Fibrous sheet material



3,628,295 Patented Apr. 3, 1962 I 3,028,296 FIBRGUS SHEET -MATERIAL James W. Adams and Leo J. Bernardin, Schofield, Wis, assignors to American Can Company, New York, N.Y., a corporation of New Jersey No Drawing. Filed Oct. 20, 1958, Ser. No. 768,061 6 Claims. (Cl. 162-146) This invention relates to the art of papermaking and particularly to the production 'of a paper web having an improved combination of tear and burst properties. More particularly, the invention relates to a paper web and a method of making the same wherein the product has an exceptional combination of high tear strength and high mullen or burst strength, being composed of uniformly intermingled fibers of polyvinyl alcohol and of papermaking cellulose.

The paper web of the present invention has particular utility in those uses for paper in which a high tear strength combined with high mullen or bursting strength are particularly desired, as for example, in bags, bread wrappers and overwrappers for boxes or cartons of frozen foods, napkin and towel stock, paper book covers and the like.

Previous attempts have been made to reinforce and improve the tear and burst properties of paper made from cellulose fibers by the incorporation of various natural and synthetic fibers in the pulp from which the web is formed but these attempts have not met with signal success. This lack of success is apparently due to the fact that these fibers, including spun glass, Dynel (a synthetic resin made by polymerization of vinyl compounds), nylon (a synthetic polyamide fiber) silk, polyacrylonitrile and the like, have no natural afiinity for the cellulose pulp fibers and do not become sufliciently bonded thereto to form a web of adequate strength without the addition of certain binding agents which increase the difiiculties and the expense inherent in the manufacture of the paper web from the pulp and detract from the overall utility of the product. Fibers of polyvinyl alcohol have not previously been considered for incorporation in papermaking furnishes since polyvinyl alcohol resins are normally considered to be water soluble and it has been believed that they would not remain as fibers after being processed in cellulose pulp slurries on papermaking equipment.

It has now been found, however, that synthetic fibers of polyvinyl alcohol are compatible with papermaking cellulose fibers and that these two types of fibers mutually assist in maintaining the individual fibers of each type asa uniform and controlled suspension in the aqueous paper making pulp slurry, preventing premature clumping of the fibers so that a coherent web of excellent formation and unusual strength properties may be obtained by passing a pulp slurry containing a mixture of these fibers over a conventional papermaking machine. From the improved strength characteristics inherent in the resulting paper web, it is evident that the synthetic polyvinyl alcohol fibers exert a strong bonding effect on the cellulose fibers.

Without wishing to be limited by the theory, we believe that the bonding efi'ect herein noted is probably due to certain intermolecular forces of the hydrogen bond type which result from the many hydroxyl groupings present tion.

for the repulsiveforces in the aqueous slurry which result in excellent dispersion of the individual fibers and, on the other hand, also account for the strong bonding force which is evident in the dried, intimately co-mingled fibers of the finished paper. The lack of these active, hydrogen-bonding groupings in such materials as nylon, polyvinyl chloride and'spun glass would account for the poor formation and inadequate strength properties of papers prepared from mixtures of these materials with cellulose pulp.

The cellulose fibers used in the present invention may be those prepared by any of the conventional methods for the production of papermaking pulp, including chemical, semi-chemical or mechanical pulp fibers prepared by, for example, the kraft, suifite or soda processes or the groundwood process. They may be bleached or unbleached stock and may be a stock which is unrefined or has been moderately refined in a beater or other refining device.

Polyvinyl alcohol fibers suitable for our use may be prepared by any suitable conventional methods such as described in-The Textile Manufacturer, December 1956, pages 636643. According to one such method, polyvinyl alcohol is dissolved in water to a concentration of 15-20% and thenextruded through a spinneret into a coagulating aqueous salt bath (suitably saturated aqueous sodium sulfate) as continuous filaments which may then be stretched or tensilized and dried at elevated temperature to make them relatively insensitive to water. Polyvinyl alcohol fibers which have not been insensitized to water by heat treatment may also be used, but require close control in drying the resulting paper sheet to avoid dissolution of the polyvinyl alcohol fibers in the water retained by the semi-dried sheet. The polyvinyl alcohol from which the fibers are formed is suitably synthesized by catalyzed reaction of acetylene and acetic acid and subsequent alcoholysis of the resulting polyvinyl acetate to form polyvinyl alcohol.

The polyvinyl alcohol fibers for use in our invention are preferably cut to a relatively uniform length which can vary from about 4 inch (about 6 mm.) up to about /2 inch (about 12 mm.) or slightly longer and can range in diameter from about 10 microns up to about 40 mion the dry weight of the furnish exhibit the improved properties which result from the practice of our inven- If the percentage of polyvinyl alcohol fibers is increased to more than about 50%, the pulp blend cannot be satisfactorily formed into paper on conventional papermaking machinery by normal papermaking procedures without the addition of special pulp dispersants and a large portion of the advantage attendant on our invention is sacrificed. Blends containing from about 5% to about 50% polyvinyl alcohol fibers may be formed into paper webs having excellent formation and unusual strength characteristics using perfectly conventional equipment and techniques, without the addition of any dispersing or bonding agents and at normal machine speeds. The formation of paper of equally desirable characteristics from blends of cellulose fibers with fibers of materials other than polyvinyl alcohol has not hitherto been attainable 41 The data in Table l indicate that significant increases in tear strength are obtained with as little as 5% of polyvinyl alcohol fibers in the furnish, and outstanding tear strengths are developed at higher polyvinyl alcohol peron conventional papermaking equipment. 6 centages. Dynel and nylon, by comparison, are signifi- Papers made from fiber blends containing relatively cantly less effective in improving the desired strength small amounts of polyvinyl alcohol fibers are of particproperties of the finished paper. ular value in the manufacture of food wrappers and the In a further series of experiments, a dilute aqueous like since they are relatively dense sheets having high slurry of softwood kraft pulp was refined to varying detear and burst strength, while papers made from pulp l0 grees by beating to various freeness values, polyvinyl blends containing from -50% polyvinyl fibers exhibit alcohol fibers one-quarter inch in length and 18 microns greater bulk while maintaining excellent tear and burst in diameter were added to the extent of 20% based on strength, and are of particular value as napkin and towel the dry weight of cellulose fibers and handsheets were stock. made and tested for tear and burst strength. In paper- In normal paperniaking operations, high burst strength It) making art, increased beating produces lower freeness or mullen is developed refining the cellulose pulp in beatvalues, increased burst strength values, and hitherto has ers, and in general, increased beating results in an increase always resulted in decreased tear strength. The results in mullen value. Beating of cellulose fibers, however, of this series of tests are recorded in Table II below. results in a serious decrease in tear strength, and hitherto it has been impossible to attain and maintain a high tear 20 strength in paper from stock which has been beaten suf- Table I1 ficiently to attain a desirably high burst strength or mullen.

By the practice of our invention, however, normal papero P 1 making cellulose fibers may be refined by beating to a a Oanadmn Freemss of euulose up Slurry degree commensurate with the production of a desired 300 150 75 value of burst strength in the finished paper sheet while Furnish the tear strength resulting from the addition of minor smngthTests on Finished Paper amounts of polyvinyl alcohol fibers 1n the papermaking pulp blend attains hitherto unprecedented values. Burst, Tear, Burst Tear Bum Tear A series of experiments was conducted in which nylon fibers, Dynel (a co-polymer of 69% acrylonitrile and 40% Polyvinyl Alcohol vinyl chloride) fibers and polyvinyl alcohol fibers of onel gb pgg? Softwood 90 A 137 440 M8 440 quarter inch in length and about 18 microns in diameter 2 S ft b d: I f% were added in varying percentages to softwood sulfate 35 Pulp 155 200 176 150 194 139 pulp in dilute aqueous suspension which had been beaten to a degree commonly referred to in the trade as 300 ml. Umts as m Table Canadian Standard freeness. Handsheets were prepared from the-pulp slurries by standard papermaking test procedures and tested for burst strength (mullen) and teen- 40 it will be noted that, contrary to the expected result strength by standard TAPPI methods. The results of as exemplified by the data on sheets made from 100% these tests are recorded in Table 1 below. softwood kraft fibers, the tear strength of the test sheets containing polyvinyl alcohol fibers remained at a very Table 1 high level despite the varying degrees of refinement by 5 beating, while the mullen was increased in the expected manner with increased beating. sy ith itic Fiber t y l Nylon It was found in further experimentation that paper havg &,,f1,gfi? ing very high tear strength could be obtained from mix- Fb a i l or Burstl Tam: Burst Tear Burst Tm tures of pape lmaking cellulose fibers with polyvinyl al =2 cohol fibers or a length as great as one-half inch or slightly more. The data recorded in Table 111, following, were 15 147 170 155 144 170 155 186 104 197 124 222 obtained on handsheets produced from blends of softl' a u 5 lg 12,3 wood sulfate papermaking pulp with polyvinyl alcohol 198 46 258 67 810 fibers of one-quarter inch, three eighths inch hand one-half 54) inch in length. The cellulose pulp was beaten to various Mullen points/ p ream sheet. ream=300 degrees of freeness prior to the addition of the polyvinyl alcohol fibers 2 Gm./10O lb. per ream sheet.

Table III Burst, Mullen Pts./l00 1b. Tear, g./1001b. Sheet PVA PVA l Softwood Sheet PVA 1 Fiber Content, Fiber Content, Percent Fiber Sulfate Percent Length, Pulp Can. In. Free, Ml.

300 144 124 106 67 170 222 303 439 810 1.76 170 175 137 95 150 240 285 440 s13 75 194 166 163 14s as 215 302 440 850 300 15a 146 128 111 05 152 241 339 539 1,098 150 142 128 121 105 7a 171 236 345 516 1,103 75 154 13s 126 105 89 1 .8 202 340 474 1,140 300 134 135 117 91 75 156 242 317 620 1,255 142 135 120 102 92 171 236 351 574 1,192 75 154 133 119 109 105 148 222 322 509 1,582

1 PVAPc-lyvinyl alcohol,

It will be noted from Table III that inclusion of up to 50% of polyvinyl alcohol fibers of three-eighths inch and one-half inch in length impart exceptionally high tear strength to paper made from cellulose fibers. It was also found that if polyvinyl alcohol fi-bers appreciably longer than one-half inch were used, or if substantially more than 50% of such fibers were used, the formation of the resulting sheet suffered and the pulp blends became more diilicult to handle on conventional papermaking equipment by normal papermaking procedures.

When papermaking cellulose pulp is beaten together with added polyvinyl alcohol fibers, a further unexpected result is obtained, in that the time required to reach a given freene-ss value for the production of paper sheets of a desired burst strength is drastically reduced, enabling significant economies in power and in the size of equipment necessary for refining a given quantity of pulp in a specified time.

Pulp blends containing up to 50% polyvinyl alcohol fibers may be processed on standard papermaking machinery at normal machine speeds using regular papermaking techniques. Blends containing more than 50% polyvinyl alcohol fibers are not as readily maintained in the uniformly distributed dilute aqueous slurry required for high quality paper production and the resulting paper 'tends to suffer from poor formation.

Papers exhibiting outstanding fold endurance have been obtained by treating the product of our invention containing blended fibers of wood cellulose and polyvinyl alcohol with from about to about 30% of a plasticizing agent such as glycerine, ethylene glycol or other low molecular weight polyhydric alcohol and calendering the plasticized paper. Fold endurance is of particular value in papers subject to repeated flexing, examples being paperbound book covers and manila file folders.

Paper handsheets were prepared in the customary manner from aqueous slurries of paperrnaking pulp containing varying percentages of polyvinyl alcohol fibers. The resulting paper sheets were impregnated with of glycerine based on the fiber weight and then subjected to a pressure of 5000 psi. at 150 C. The fold endurance of the resulting pressed sheets was measured by the TAPPI standard method No. T423m-50 using a fold tester of the type developed at the Massachusetts Institute of Technology. The results of these tests are recorded in Table IV, following.

Remarkable increases in fold endurance such as those recorded in Table IV are obtained when the sheets of our invention are treated with from 10 to 30% based on total fiber weight of a polyhydric alcohol plasticizer followed by pressing or calendering at elevated temperatures.

Having now disclosed several of the preferred modifications of our invention, it vis evident that various other modifications may be made within the spirit of our invention. We do not wish to be limited in the interpretation of our invention except as distinctly set forth in the appended claims.

We claim:

1. Paper sheet material of improved burst and tear strength comprising a blend of papermaking cellulose pulpfibers and a minor amount of polyvinyl alcohol fibers.

2. Paper sheet material of improved burst and tear strength comprising a blend of papermakin-g cellulose pulp fibers and from about 5% to about 50% of fibers of tensilized polyvinyl alcohol.

3. Paper sheet material of improved burst and tear strength comprising a blend of papermaking cellulose pulp fibers and from about 5% to about 50% of Waterinsensitized polyvinyl alcohol fibers.

4. Paper sheet material of improved burst and tear strength comprising a blend of paperrnaking cellulose pulp fibers and from about 5% to about 50% of waterinsensitized polyvinyl alcohol fibers, said polyvinyl alcohol fibers having a fiber length substantially greater than the average length of said cellulose fibers.

5. Paper sheet material of improved burst and tear strength and improved fold endurance comprising a blend of papennaking cellulose fibers, a minor amount of waterinsensitized polyvinyl alcohol fibers and from about 10% to about 30% based on the total fiber weight of a low molecular weight polyhydric alcohol plasticizer.

6. Paper sheet material as described in claim 5 wherein the polyhydr-ic alcohol plasticizer is glycerine.

References Cited in the file of this patent UNITED STATES PATENTS 1,829,585 Dreyfus Oct. 27, 1931 2,648,635 Brown et al Aug. 11, 1953 2,962,762 Hartmann et a1. Dec. 6, 1960 FOREIGN PATENTS 1,083,697 France June 20, 1954- 572,962 Great Britain Oct. 31, 1945 OTHER REFERENCES The Textile Manufacture, December 1956, pages 636- 

1. PAPER SHEET MATERIAL OF IMPROVED BURST AND TEAR STRENGTH COMPRISING A BLEND OF PAPERMAKING CELLULOSE PULP FIBERS AND A MINOR AMOUNT OF POLYVINYL ALCOHOL FIBERS. 